Recently, many bicycles have been given a greater number of speed steps: A front gear connected to a crank and a rear gear connected to a rear hub each comprises a plurality of sprockets having different numbers of teeth.
These front and rear gears are connected by a chain. A rotational momentum generated by pedalling is received by the crank and front gear, and then transmitted via the chain to the rear gear, thereby turning the rear wheel for the bicycle to move forward.
A speed shifting is made while the multiple sprockets are rotating; the chain is forcibly pressed laterally by a derailleur off a track before engaging with the multiple sprocket, thereby disengaging the chain from a sprocket for engaging with a desired sprocket. Efficiency of this speed shifting is determined by how quickly the chain is disengaged from the sprocket with which it engages presently and how quickly the chain is engaged with the desired sprocket.
The chain is given a predetermined amount of tension by the derailleur. Because of this tension, a chain shifting from a diametrically larger sprocket to a diametrically smaller one can be performed relatively easily: When the chain is disengaged from the diametrically larger sprocket, the tension automatically causes the chain to fall on the diametrically smaller sprocket.
However, when the chain is shifted from a diametrically smaller sprocket to a diametrically larger sprocket, disengaging the chain from the diametrically smaller sprocket is not enough. The chain must be raised against the tension to a height where the chain can be engaged with the diametrically larger sprocket.
For this reason it is generally recognized that a chain shifting from a diametrically smaller sprocket to a diametrically larger sprocket is more difficult than a chain shifting from a diametrically larger sprocket to a diametrically smaller sprocket.
When there is a large difference in the number of teeth between a diametrically larger sprocket and a diametrically smaller sprockets, there is also a large difference in diameter between the two sprockets, resulting in a long portion of the chain to be in transitional state between the diametrically smaller sprocket and the diametrically larger sprocket, making difficult to guide the chain stably to a predetermined teeth furrow on the diametrically larger sprocket.
In an attempt to solve this problem, an invention disclosed in the Japanese Utility Model Publication 63-2392 for example, proposes a multiple sprocket assembly, wherein a side wall of the diametrically larger sprocket facing the diametrically smaller sprocket is provided with a guide wall for supporting the chain to move along a tangential line extending from a furrow center of a pair of teeth on a diametrically smaller sprocket.
By providing the guide wall, it becomes possible to support an intermediate portion of the transitional portion of the chain, thereby securely shifting the chain to the diametrically larger sprocket.
According to the multiple sprocket assembly described in the above Japanese Gazette, the chain moves along a straight line from a teeth furrow center on a diametrically smaller sprocket to another teeth furrow center on a diametrically larger sprocket, and the guide wall is provided along this straight transitional path for supporting the chain's link plate.
However, in this multiple sprocket assembly, the chain having been raised close enough to a tooth of the diametrically larger sprocket must be re-oriented circumferentially of the diametrically larger sprocket before it can engage with the diametrically larger sprocket.
Thus, the chain is subject to a substantial bend along a vicinity of a link plate which is about to engage with the diametrically larger sprocket.
As a result, a link plate running immediately ahead of the link plate to be engaged firstly with the diametrically larger sprocket is pressed onto a side surface of the diametrically larger sprocket. At the same time, the chain's lateral deflection concentrates on the vicinity of the link plate subject to the bend. Thus, the chain and the sprocket teeth are subject to a substantial force.
Especially in the case of the front gear, when the above-described state is followed by further input of driving force, it is likely that a tooth of the diametrically larger sprocket may wedge into a pair of overlapping link plates, damaging the chain or the sprocket's tooth.